The study is the first to compare time served in public and private prisons, and suggests financial reasons exist for keeping people behind bars.

A new study reveals that private prisons in the United States keep people locked up two to three months longer than public prisons, and are just as likely to see people commit another crimes after their release, according to media reports Thursday.

Instead, what it indicates is that there may be a financial incentive for operators of private prisons to maximize the number of days each prisoner serves and keep prisons full, since private prisons are paid on the basis of each occupied bed.

According to Anita Mukherjee, an assistant professor of actuarial science, risk management and insurance at the University of Wisconsin who authored the study, this system may not be in the best interest of the state.

“The number of days a prisoner serves relates directly to the cost of housing that inmate, so if inmates sent to private prisons somehow serve longer terms, this undermines the very cost benefit that makes private prisons attractive relative to public prisons," she says.

In Mississippi, for example, the state is contracted to pay private operators an average US$50 per bed occupied, so an extra three months of incarceration would cost the state $3,000 per inmate – cutting into about half the projected cost savings purportedly offered by private prisons.

Mukherjee also found that inmates in private prisons receive twice as many infractions while incarcerated, what are usually used by the parole boards to assess ones eligibility for early release.

The system also creates unfairness for prisoners who end up “serving more time simply because they were randomly assigned to a private prison instead of a public one,” said Mukherjee.

The private prison system in the U.S. is a US$5 billion industry, and represents some 10 percent of all incarcerations in the country. The system has been constantly criticized by human rights organizations who say the companies are profiting off the incarceration and suffering of individuals.